1. Technical Field
The present invention relates to an illumination system for applying light onto an object such as a liquid crystal device, a liquid crystal device incorporating the illumination system, and an electronic device incorporating the liquid crystal device.
2. Related Art
Various electronic devices such as portable phones and personal digital assistants widely use a liquid crystal device as a display for visually displaying various information of the electronic devices. The liquid crystal devices need sun light, room light, or an illumination system because they are not selfluminous devices.
The illumination systems used in liquid crystal devices generally include a light source that emits light and a light guide that converts the light from the light source to planar light and that outputs it. A frequently used light source is a light emitting diode (LED). Light emitted from LEDs may cause light portions and dark portions on the light guide illuminated by the light because of its directivity.
Known illumination systems use a light guide that has a plurality of protrusions or recesses on the light-incident surface (see JP-A-2006-59559, pp. 5-6, FIG. 3, JP-A-2006-49192, p. 5, FIG. 2, and JP-A-2006-4645, p. 4, FIG. 3). Those illumination systems refract light by the protrusions or recesses to diffuse the light to a wide range of the light guide, thereby distributing the light exiting from the light source evenly.
It is desirable that the light source be disposed in a position at which light of even intensity is applied to the widest possible area in the plane of the light guide. However, the light guide cannot always be disposed in a desired location, because liquid crystal devices are composed of various elements. In such a case, the uneven distribution of light incident on the light guide may cause high lighted portions by excessive light and low lighted portions by little light.
FIG. 12A shows an example of known illumination systems. The illumination system, dented at 201, includes a light guide 203 and light sources 202L and 202R. The light-incident end face 203a of the light guide 203 has no evenness. When the two light sources 202L and 202R are placed at positions Pr in the X direction along the width of the light guide 203, light of even intensity can be applied to the widest possible area of the light-exiting surface 203b of the light guide 203. The positions of the light sources 202L and 202R at which light of even intensity can be applied in the widest possible area of the light-exiting surface 203b of the light guide 203 are hereinafter referred to as “reference positions Pr”. This is not limited to the illumination system 201 of FIG. 12A as long as the light incident end face 203a is planar.
FIG. 12B illustrates the distribution of light emitted from the two light sources 202L and 202R, which are disposed off the reference position Pr closer to each other, and entering the light guide 203. The two light sources 202L and 202R are sometimes disposed off the reference positions Pr because of necessity of the internal structure of an electronic device that incorporate the illumination system 201 as one component.
Referring to FIGS. 12A and 12B, the light emitted from the light sources 202L and 202R enters light radiation areas αL and αR. The traveling direction of the light (that is, the orientation of the light radiation areas αL and αR) is substantially perpendicular to the side end face 203a of the light guide 203. In this specification, the traveling direction of the light is sometimes referred to as “the optical axis”.
In FIG. 12A, the light sources 202L and 202R are arranged so that the light radiation area αL of the light emitted from the light source 202L and the light radiation area αR of the light emitted from the light source 202R are not overlapped. On the other hand, in FIG. 12B, the light radiation areas αL and αR are partly overlapped because the interval between the light source 202L and the light source 202R is small. In this case, the luminous intensity may become high in the area β where the light radiation areas αL and αR overlap, causing uneven luminous intensity in the plane of the light guide 203.
To solve the above problem, it may be effective to change the traveling direction or the direction of the optical axis of the light incident on the light guide 203 in correspondence with the positions of the light sources 202L and 202R. However, the illumination systems disclosed in JP-A-2006-59559, JP-A-2006-49192, and JP-A-2006-4645 cannot change the direction of the optical axis, although they can diffuse the light incident on the light guide. Thus, the related-art illumination systems cannot prevent the light emitted from different light sources from overlapping in the light guide to cause uneven luminous intensity in the plane of the light guide.